The further the tool does its work from the spindle, the more room for error. Deep pockets and cavities usually signify that a part is nearing completion, making the smallest of errors almost impossible to recover from and very costly.
Machining at higher speeds can save time and money, however, increasing the speed also means increasing the inherent risk in the situation. To maximize speed potential, a holistic approach that considers tool, holder, machine and operator is essential.